In recent years, mechanical drive systems in vehicles are gradually replaced with electrical drive-by-wire systems, for space efficiency, ease of assembly, controllability and the like, in which the mechanism of the drive system is driven by motors. One of the examples of such replacement is found in a field of automatic transmission, or more practically, a range switching mechanism driven by a motor, as disclosed in a patent document, JP 2004-308846 A (patent document 1). In such a system, the drive control of the motor is performed based on an encoder count (i.e., by counting the number of pulse signals from an encoder in the motor), and the motor is rotated to a target rotation position that corresponds to the target range (i.e., the motor is drive to the target encoder count), for the switching of the shift ranges. Here, the shift ranges may also be understood as the shift positions of the automatic transmission.
In the above-described system or the like, the reference position of the rotation position of the motor is learned for the rotation control of the motor by an abutment control, in which the motor is rotated to operate the range switch mechanism to the limit position of the movable/rotatable range (i.e., to one of two walls on both ends of rotatable range), for the learning of such position as a reference position, as is disclosed in the patent document 1. However, in such an abutment control, the rotation transmission mechanism is resiliently twisted by the motor torque when the motor is rotated to move the range switch mechanism to the limit position, in which a detent spring or the other parts may be twisted or warped or pulled (i.e., resiliently deformed), from the original shape, for example. Therefore, in the above-described system in the patent document 1, after the motor is rotated to the limit position, the motor is rotated in a reverse direction that is an opposite rotation direction relative to the rotation in the abutment control, for the motor rotation position of rotating the range switch mechanism to the limit position to be returned by a preset rotation amount from such position and for the resolving/cancelling of the resilient deformation of the rotation transmission system that has been rotated to the limit position. The power supply to the motor is stopped after such a return control for returning the rotation position of the motor from the limit position.
In recent years, a fuel efficiency of the vehicle and improvement in reliability are in high demand. However, in the technique described in the patent document 1, the return control of the motor after the abutment control uses an electric power supplied from the battery, for the rotation of the motor in the reverse direction, which means that the electric power consumption and heat generation are both increased from such a return control of the motor and the fuel efficiency and reliability of the system may thus be deteriorated.